Runners make their way through the snow in front of the Lincoln Memorial on Thursday, February 13 in Washington. The harsh winter conditions this year have tested runners, but been have been good for the running industry.

Brandon Park once worked in the freezers at a food warehouse. So it’s not the sub-zero weather in his suburb north of Dayton, Ohio, that’s kept him from running outdoors this winter.

“It was a battle just to stay upright” outdoors,” he says. “Running shoes just don’t keep traction.”

Park, 28, is one of endless numbers of runners struggling through this winter’s bitter cold and heavy snow. But to others—indoor health clubs and apparel makers who sell hats, gloves, trail shoes, and layers, for instance—the weather is surprisingly good for business.

“Bad weather is good for fitness clubs, there’s no denying that,” says Mark Daly, spokesman for Minnesota-based Anytime Fitness, which he said has added more members per club in the first five weeks of this year than at any time in its history.

“As somebody who visits my local club nearly every morning I can tell you that, yes, the treadmills are packed,” said Daly. “The weather is having a definite impact.”

Runners have dealt with some tough conditions this winter. A report on Weather.com discussing the much talked about polar vortex says, "One of the coldest Arctic outbreaks in two decades has plunged into country, bringing bitterly cold temperatures to the Midwest, South and East." This week, Winter Storm Pax added to the brutal winter, dropping as much as 28 inches of snow in some places along the East coast.

Susan Irwin of Lexington, Nebraska, joined a gym this winter, too.

“If there’s anything on the ground, I pay too much attention trying not to slip,” says Irwin, 43.

She’s had a lot of company, she says.

On one subzero weekday after 8 a.m., she says, “All the treadmills were taken. Usually it’s pretty quiet at that hour, which is after people go to work.”

All of that traffic has even further swelled the traditional mid-winter rush at gyms, says Meredith Poppler, spokeswoman for the International Health, Racquet & Sportsclub Association, which reports that, even during mild winters, twice as many people join gyms in January than in July. (Forty-two percent use treadmills, making them the second-most used pieces of equipment, after resistance machines.

“Obviously this weather puts everybody indoors,” Poppler says.

“We’re telling people, you should be working out indoors. It’s safer, you have motivation from other people around you,” she says. “Obviously a treadmill should never replace the real reason you run in the first place, which is to be outside.”

Their insistence on coping with the elements, no matter how severe, is why runners are also fueling hot sales of running hats, gloves, layers, and cold-weather shoes, manufacturers say.

Running stores have seen an increased demand for winter running gear. Dan Fitzgerald, the co-owner/founder of South End Athletic Company and Heartbreak Hill Running Company said his store ran out of balaclavas this year for the first time in the four and a half years he's owned the stores. He's also seen an uptick in the sale of waterproof, winterized shoes, and Nike Element Sheild tights have sold particulary well, too.

"We've managed demand pretty well," Fitzgerald said. "The vendors have product in stock. But we would've loved to get more balaclavas from Brooks, but Brooks doesn't have them."

Even before the holidays, Under Armour “certainly saw some benefit” to sales of its Fleece and ColdGear lines from the weather, chairman and CEO Kevin Plank told a conference call with financial analysts.

That company had the good timing to introduce its new extreme ColdGear Infrared products just as winter started raging, and a spokeswoman says sales have been “astronomical." Chief financial officer Brad Dickerson told analysts they helped fuel Under Armour’s 35 percent growth in the quarter that ended December 31.

“You could easily say maybe a couple percentage points of that was due to cold weather,” Dickerson said. “We definitely realize that there's some tailwind of weather, and it makes our results a little bit better.”

A Nike spokeswoman says it, too, has seen a rise in sales of reflective and water-repellant shoes designed for use in the cold and dark, and in jackets, zips, and especially reflective tights.

Alicia Clouse’s Christmas list accounts for at least some of that, she says. Clouse, who also is a runner, picked this winter to move from Arizona to Montana.

“I asked for running clothes for Christmas and just last week bought trail shoes, because running in the snow with my regular shoes, they collect snow in the heels,” she says.

Clouse, 25, who is training for a half marathon in June and is scheduled to run a marathon in September, says she’s stocked up on long-sleeved shirts, a jacket, wool socks, a headwrap, and running gloves.

Even with all of that, she’s been forced inside on the coldest days.

But she looks at the bright side.

“It makes the long runs outside even better,” Clouse says. “Running in cold and snow is a blast. I love it. Which is weird, because I grew up in Arizona.”